Why devRant not support comment editiing?

It's not a bug, it's a feature

Because it's only a few months old but already has thousands of active users. It which has a web, ios and android app, and all of that is being written and maintained by two guys.

also @dfox likes polish things where as I'm like, goddamniticouldhackthistogetherinliketwodays.gif

Also, markdown w/ syntax highlighting would be awwwwwesome

Because it would be hard to make distinction between editing for typos or editing because you plan to change a message completely after someone replied.
Maybe an edit during the first minute might be an option, but I don't think it's a priority.

I love the way discourse does it, a discrete comment icon in the corner opens a modal which lets you browse through revisions and even see a git-style diff

I mean, you can't get any better than that. And, totally not overkill.